The present invention relates to electron beam devices and more particularly, to an electron beam device which can suppress the contamination of a differential exhaust diaphragm regardless of the value of a probe current.
In an electron beam device represented by a typical scanning electron microscope, various sorts of observation condition parameters are set according to a specimen or observation conditions, and a user observes the enlarged image of the specimen. The observation condition parameters include, for example, acceleration voltage, the emission current of an electron gun, condenser lens conditions, working distance (a distance between an objective lens and a specimen), conditions of a system for detecting a signal for specimen observation, and objective diaphragm aperture diameter.
Other electron beam devices are used to analyze a very small region on a specimen by combining with various types of analysis devices. The analyzing method includes, for example, an energy-dispersive X-ray (EDX) spectroscopy, wavelength-dispersive X-ray (WDX) spectroscopy, and an electron backscatter diffraction pattern (EBSP) method.
In the EDX spectroscopy, spectroscopy is carried out by directly detecting a characteristic X ray generated from a specimen with use of a semiconductor detector and by converting the detected X ray to an electric signal. A pulse current is generated in proportion to the energy of the detected characteristic X ray, and the pulse current is sorted and measured by a multichannel crest analyzer.
In the WDX spectroscopy, spectroscopy is carried out by separating and detecting an X ray having a specific wavelength based on Brag reflection at spectroscopic crystal. The wavelength of the characteristic X ray is measured from the diffraction angle of the X ray Brag-reflected by the spectroscopic crystal to identify the type of an element.
In the EBSP method, since a Kikuchi pattern created by inelastic backscatter electrons from a specimen varies with the orientation of the specimen, the pattern is analyzed by scanning a specimen with use of an incident electron probe to obtain an image distributed in the crystalline orientation of the polycrystalline specimen.
When these analysis methods are employed, it is necessary to make the value of a current of an electron beam to be irradiated on the specimen larger than that merely when the enlarge image of the specimen is obtained. For example, a probe current Ip for high resolution observation may be set to have a value of several pA to tens of pA. However, the probe current is required to have a value of hundreds of pA in the EDX spectroscopy, whereas, the probe current is required to have a value of about tens of nA in the WDX spectroscopy.
It is generally considered that the more the charge of an irradiated electron beam increases or the lower the vacuum level of a site of a specimen having the electron beam irradiated thereon is (or the higher a remaining internal pressure is), the higher the contamination level of an element such as each diaphragm in an electron beam device is. In other words, the interior of the device when analyzing a specimen becomes more contaminated than when the device observes the enlarged image of the specimen.
In the electron beam device, in general, the interior of a mirror section having a specimen mounted therein is kept at a high vacuum level, and the interior of an electron gun section including a cathode for emitting an electron is kept at a vacuum level lower than that of the mirror section. In order to pass an electron beam through the device, the interior of the electron gun section communicates with the interior of the mirror section via a small aperture known as a differential exhaust diaphragm.
Accordingly, the aperture diameter of the differential exhaust diaphragm is set to have preferably a minimum value from the viewpoints of having no influences on an optical system and suppressing movement of a substance such as a gas between the electron gun section and the mirror section. In other words, the contamination of the differential exhaust diaphragm can lead to a cause of immediately reducing the optical performance of the electron beam device.
In order to avoid the above problem, there is proposed a prior art variable diaphragm device for an electron microscope or the like, wherein a long diaphragm plate is previously wound around one shaft, winding of the diaphragm plate by rotating the other shaft causes one of diaphragm holes made in the surface of the diaphragm plate to traverse an opening or aperture made in a case, so that, even when one diaphragm aperture is contaminated, another diaphragm plate aperture can be replaced therewith (refer to JP-A-11-233053, paragraphs 0012-0014, FIG. 2).
Also proposed is a charged particle beam device which, when the total amount of charges of a primary electron beam hit on a diaphragm plate exceeds a predetermined reference value, determines that the contamination of a diaphragm aperture exceeded such an allowable limit value as not to have influences on an image quality, and the current diaphragm aperture is replaced with new one by means of an actuator (refer to JP-A-2004-95459, paragraph 0021, FIG. 2).
Further proposed is a scanning electron microscope which comprises a diaphragm plate having a diaphragm aperture and a thick plate having an aperture section and laminated on the diaphragm plate, and in which a voltage applying means applies a high voltage to the diaphragm plate for flashing to remove a contaminant deposited on the diaphragm plate (refer to JP-A-2000-200574, paragraph 0019, FIG. 1).
However, the above prior art variable diaphragm device (see JP-A-11-233053) for an electron microscope or the like has a problem that, since the diaphragm plate is slid against the case, a gas or the like tends to leak at the sliding part and thus the device is not suitable as a differential exhaust diaphragm.
The above prior art charged particle beam device (JP-A-2004-95459) has a mechanism for switching between diaphragm holes with use of the actuator. However, the device also has a problem that, since a leakage of a gas or the like tends to take place in the switching part of the mechanism and the differential exhaust diaphragm has a complex structure, it is difficult to employ the mechanism in the differential exhaust diaphragm.
In the scanning electron microscope (JP-A-2000-200574) of the above prior art, in order to effectively prevent the contamination of the diaphragm plate, the thick plate is required to have a large thickness; whereas, in order to apply the high voltage for flashing, the periphery of the diaphragm plate is required to have a high insulation structure. Accordingly, the prior art microscope is disadvantageous in that it is difficult to employ such a structure for the differential exhaust diaphragm at which a high vacuum atmosphere comes into contact with a ultra high vacuum atmosphere and which is required to keep an optically accurate positional relationship.
Any of the aforementioned prior arts is basically arranged, only after the diaphragm aperture is contaminated, so as to take ex-post-facto counter-measures against it. In particular, when the probe current is large, the contamination of the differential exhaust diaphragm is disadvantageously increased. For the purpose of solving the above problem, from the viewpoint of avoiding reduction in vacuum level or fluctuations in optical performance caused by provision of an excessive margin to the aperture diameter of the diaphragm hole, simplifying the structure, and facilitating the operation, maintenance and management; there has been demanded a technique not for performing exchange or contamination removal each time that the differential exhaust diaphragm is contaminated but for suppressing the deposition itself of a contaminant deposited on the diaphragm.